174 MR JAMES MURRAY ON 



The spurs are very strongly curved. in lateral view they look like a drag-anchor ; 

 in dorsal view the points are often further forward than the base. They are very 

 slightly movable, and these positions are maintained. The toes are of quite unusual 

 structure. Close under the spurs there projects from the back of the terminal foot- 

 segment a short cylindrical joint ; from this issue two short, curved, pointed toes looking 

 like a second pair of spurs ; the segment then forks and bears the very large terminal 

 toes. As was the case with the related P. laticornis (39), the toes are seldom retracted, 

 but remain extended after the new grip has been taken. 



The species has now been definitely ascertained to be parasitic on Gammarus, both 

 in Loch Tay and in St Mary's Loch. 



It has been sufficiently distinguished from P. laticeps above. The remarks made 

 under P. laticeps about Callidina parasitica, Giglioli, serve to distinguish P. hamata 

 also from that species. The lack of eyes, besides other characters, separates it from 

 P. laticornis and P. commensalis. No other species known to me comes near enough 

 to need detailed comparison. 



Callidina. — Oviparous ; toes three, or united to form a sucker. This genus also is 

 divided into three sections, indicated by the letters A, B, C. The first is a very 

 natural group, the other two are not so certainly distinct ; the symbiotic foot may have 

 been independently acquired by diverse animals, and Zelinka's various symbiotic 

 species do riot seem to be particularly closely related otherwise. 



A. Food moulded into pellets. 



B. Toes three, distinct, no pellets. 



C. Toes united to form a sucker. 



These subdivisions are rendered necessary by the diverse structure of the numerous 

 species. Group A, the pellet-makers, is one of the largest natural groups within the 

 order, many species being still undescribed. 



Although all conform to a uniform type of structure, there is great diversity of 

 external form, the most aberrant being probably C. cornigera, C rceperi, and 

 C. hexodonta. 



These last two are the only species of the genus Callidina, as here defined, which 

 possess eyes. As these are placed as in the genera Rotifer and Philodina respectively, 

 this may be an indication that the group of the pellet-makers is of more than generic 

 value. 



In Mr Bryce's projected revision of the classification of the Bdelloids, I understand 

 that the three subdivisions of the genus Callidina here adopted will be among the 

 groups elevated to generic rank. 



C. hexodonta (Bergendal) (2). (Plate III. fig. 13.) 



Mr Bryce has suggested, very plausibly, that this may be Philodina collaris of 

 Ehrenberg. As there is some doubt about it, while it is pretty certainly the Philodina 



